In co-pending U.S. patent application Ser. No. 419,705 filed on Sept. 20, 1982, a technique was disclosed that enables an optical designer to select compatible optical materials for the refractive elements of an optical system that is to be color-corrected at a specified number of wavelengths. A technique disclosed in co-pending U.S. patent application Ser. No. 260,106 filed on Oct. 20, 1988 enables the designer of a lens system that is to be color-corrected at a specified number of wavelengths to include one or more liquid lens elements among the lens elements of the system.
In co-pending U.S. patent application Ser. No. 282,665 filed on Dec. 12, 1988, examples were disclosed of apochromatic lens triplets of a type in which a liquid lens element is contained between two geometrically identical glass lens elements facing in opposite direction. In co-pending U.S. patent application Ser. No. 345,793 filed on May 1, 1989, examples were disclosed of apochromatic lens systems of a type in which a number of identical glass lens elements and a liquid lens element are used to achieve high performance as indicated by a change in focus of less than one-quarter wavelength over the visible spectrum.
In co-pending U.S. patent application Ser. No. 379,386 filed on July 13, 1989, examples were disclosed of color-corrected lens systems of a type in which identical plastic lens elements and a liquid lens element, all of which have spherical surfaces, are used to achieve high performance. Surface sphericity is highly desirable for glass lens elements, because the process of grinding (or chemically etching) and polishing a glass lens surface to an aspherical configuration is generally very costly. However, since plastic lens elements can be formed by injection molding, it is relatively inexpensive to provide aspherical surfaces for plastic lens elements.
The aspherizing of lens surfaces is a well-known design expedient for improving performance of a lens system with respect to the monochromatic aberrations, particularly spherical aberration. However, for a lens system that is to be operated over a broad wavelength band (e.g., the visible spectrum), the aspherizing of a lens surface introduces chromatic variations of the monochromatic aberrations, especially chromatic variation of spherical aberration, which ordinarily causes a significant degradation in performance.